The Collaborative International Dictionary of English v.0.48:

Watch \Watch\ (w[o^]ch), n. [OE. wacche, AS. w[ae]cce, fr.
   wacian to wake; akin to D. wacht, waak, G. wacht, wache.
   [root]134. See Wake, v. i. ]
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   1. The act of watching; forbearance of sleep; vigil; wakeful,
      vigilant, or constantly observant attention; close
      observation; guard; preservative or preventive vigilance;
      formerly, a watching or guarding by night.
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            Shepherds keeping watch by night.     --Milton.
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            All the long night their mournful watch they keep.
                                                  --Addison.
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   Note: Watch was formerly distinguished from ward, the former
         signifying a watching or guarding by night, and the
         latter a watching, guarding, or protecting by day
         Hence, they were not unfrequently used together,
         especially in the phrase to keep watch and ward, to
         denote continuous and uninterrupted vigilance or
         protection, or both watching and guarding. This
         distinction is now rarely recognized, watch being used
         to signify a watching or guarding both by night and by
         day, and ward, which is now rarely used, having simply
         the meaning of guard, or protection, without reference
         to time.
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               Still, when she slept, he kept both watch and
               ward.                              --Spenser.
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               Ward, guard, or custodia, is chiefly applied to
               the daytime, in order to apprehend rioters, and
               robbers on the highway . . . Watch, is properly
               applicable to the night only, . . . and it begins
               when ward ends, and ends when that begins.
                                                  --Blackstone.
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   2. One who watches, or those who watch; a watchman, or a body
      of watchmen; a sentry; a guard.
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            Pilate said unto them, Ye have a watch; go your way,
            make it as sure as ye can.            --Matt. xxvii.
                                                  65.
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   3. The post or office of a watchman; also, the place where a
      watchman is posted, or where a guard is kept.
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            He upbraids Iago, that he made him
            Brave me upon the watch.              --Shak.
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   4. The period of the night during which a person does duty as
      a sentinel, or guard; the time from the placing of a
      sentinel till his relief; hence, a division of the night.
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            I did stand my watch upon the hill.   --Shak.
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            Might we but hear . . .
            Or whistle from the lodge, or village cock
            Count the night watches to his feathery dames.
                                                  --Milton.
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   5. A small timepiece, or chronometer, to be carried about the
      person, the machinery of which is moved by a spring.
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   Note: Watches are often distinguished by the kind of
         escapement used, as an anchor watch, a lever watch,
         a chronometer watch, etc. (see the Note under
         Escapement, n., 3); also, by the kind of case, as a
         gold or silver watch, an open-faced watch, a
         hunting watch, or hunter, etc.
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   6. (Naut.)
      (a) An allotted portion of time, usually four hour for
          standing watch, or being on deck ready for duty. Cf.
          Dogwatch.
      (b) That part, usually one half, of the officers and crew,
          who together attend to the working of a vessel for an
          allotted time, usually four hours. The watches are
          designated as the port watch, and the starboard
          watch.
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   Anchor watch (Naut.), a detail of one or more men who keep
      watch on deck when a vessel is at anchor.

   To be on the watch, to be looking steadily for some event.


   Watch and ward (Law), the charge or care of certain
      officers to keep a watch by night and a guard by day in
      towns, cities, and other districts, for the preservation
      of the public peace. --Wharton. --Burrill.

   Watch and watch (Naut.), the regular alternation in being
      on watch and off watch of the two watches into which a
      ship's crew is commonly divided.

   Watch barrel, the brass box in a watch, containing the
      mainspring.

   Watch bell (Naut.), a bell struck when the half-hour glass
      is run out, or at the end of each half hour. --Craig.

   Watch bill (Naut.), a list of the officers and crew of a
      ship as divided into watches, with their stations.
      --Totten.

   Watch case, the case, or outside covering, of a watch;
      also, a case for holding a watch, or in which it is kept.


   Watch chain. Same as watch guard, below.

   Watch clock, a watchman's clock; see under Watchman.

   Watch fire, a fire lighted at night, as a signal, or for
      the use of a watch or guard.

   Watch glass.
      (a) A concavo-convex glass for covering the face, or dial,
          of a watch; -- also called watch crystal.
      (b) (Naut.) A half-hour glass used to measure the time of
          a watch on deck.

   Watch guard, a chain or cord by which a watch is attached
      to the person.

   Watch gun (Naut.), a gun sometimes fired on shipboard at 8
      p. m., when the night watch begins.

   Watch light, a low-burning lamp used by watchers at night;
      formerly, a candle having a rush wick.

   Watch night, The last night of the year; -- so called by
      the Methodists, Moravians, and others, who observe it by
      holding religious meetings lasting until after midnight.


   Watch paper, an old-fashioned ornament for the inside of a
      watch case, made of paper cut in some fanciful design, as
      a vase with flowers, etc.

   Watch tackle (Naut.), a small, handy purchase, consisting
      of a tailed double block, and a single block with a hook.
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WordNet (r) 3.0 (2006):

watch glass
    n 1: laboratory glassware; a shallow glass dish used as an
         evaporating surface or to cover a beaker
    2: a protective cover that protects the face of a watch [syn:
       crystal, watch crystal, watch glass]